In terms of trusting Fabsol, there's not really any reason to going by track record alone. The mod is bloating to the point of being a logistical problem. It's too large for tmodder and starting to use too much memory for a 32 bit program. If you really examine what's been added over the years, Calmity has been finished and over with for a long time. The bosses, all the way up to Calamitas, have been in the mod for a long time, and while the post-Moonlord bosses have been rebalanced, it's been mostly just to make them more tedious.
So what's been added that's worth noting?
Piss Ocean
Well, we've got the Piss Ocean. That's big; it's a big player-killing waste of time except for two items - the teardrop that controls weather and the siren light pet. The weapons in the Piss Ocean get replaced shortly after the Wall of Flesh is defeated, and many of those weapons aren't specifically amazing against the Wall of Flesh either. They're in the upper tier of pre-hardmode, but they're also randomly generated and thus not worth thinking about if you don't plan to hop to different world gens to find them. Additionally, it's obnoxious because although Calamity adds some water accessories which might validate the biome, the Piss Ocean has its own special tryhard mechanics that cause the player to lose breath anyway.
The Piss Ocean also contains notable enemies such as the Mauler Shark and the Scourge, both of which appear pre-hardmode and can be killed pre-hardmode, but don't offer rewards until hardmode. The Mauler Shark, which is more powerful than most bosses, doesn't even drop an item until after Polterghast, which is so late as to be inconsequential. Why does this enemy, which appears at the very start of the game, not drop anything useful until I'm fighting DoG? The whole biome just feels like this - an inconsistent series of nonsensical choices, and we were just barely fortunate enough to get anything useful out of it. It could be made into a separate mod, but if it weren't bundled with Calamity I doubt many players would feel it adds a lot to the experience. The Piss Ocean is an excellent candidate to remove bloat.
Sky Clutter
Planetoids, a recent addition, also add nothing of any real value. Mostly acting as convenient redundancies, Planetoids offer easy mushroom biomes and queen bee larvae. Personally, I find that over the span of the game, I usually run into a natural mushroom biome anyway, and you don't really need a mushroom biome until you need Truffle. As for the Queen Bee, exploring and searching for things is part of Terraria, and if you find even a single hive you'll have enough resources to construct and Abeemination. You really don't need to extend the sky and clutter it up with land masses for that.
I thought maybe something would come of the Sky Clutter, but like the Piss Ocean, there's not really anything of value to justify the extra file size. Actually it offers less than the Piss Ocean.
Calamity Biome
It's been in the mod for a long time, but like a lot of things in Calamity, it's just "[x], but TRYING HARDER". It's really just the Underworld, but more tryhard. It's a small biome, there's nothing consequential in it except for some ore which can be skipped. You summon an optional boss there, which I used to like fighting until the addition of the Brimlings. Now that the Brimstone Elemental has little sidekicks that get in the way, I just ignore it. It's not like she drops anything I want until post-Moonlord anyway. Honestly, you could cut the whole Calamity biome and use the Underworld as it is. If you feel it needs hardmore ore, then propagate the ore in the Underworld.
"Lore"
Goodness, how embarrassing. Was this really necessary? And did everything, even the deserts need to be connected directly to Calamity's NPCs? I neither need nor want speculation for things like "Cthulhu's Eye" because by being related to that eldritch horror it's implied to be beyond my understanding in the first place. It's just a Thing That Cannot Be. But then we have these little blurbs that make it feel like you have a hard time accepting your bosses are meant to be defeated. Adding text to make it clear that the only reason we beat Cryogen is "because he got weak" feels, again, like TRYING HARDER. Like we beat this boss, so now the mod tries really hard to convince us that the boss is totally way bigger and badder than the experience would imply.
Rogue Class
Is it necessary? "Just trust me guys". I mean, eh. Okay. It's not that I trust you - because I don't - it's that it's your mod and you'll do with it whatever you do. You don't need my trust and if you were overly concerned with player sentiment I'm sure you'd have waited until you had something to show. There are other mods that make use of the "throwing" class and you're breaking compatibility, but you already knew that, and I suspect "Rogue" is going to be "Throwing, but TRYING HARDER".
Post-Moonlord Bosses
And finally, not something you can cut, but something that's being changed every new iteration of the mod. Not really to make them more fun or more balanced, but to TRY HARDER. DoG can turn invulnerable, Dragon Fishron recovers his health and forces you to fight him twice. Calamitas gets a cage and then vomits up a bunch of random projectiles into it - something which a lot of the post-Moonlord bosses do.
There's all this talk of adding in even stronger bosses, like Yharim, but it's not a wonder it hasn't happened yet if DoG and Calamitas indicate what their fights would be like. I usually don't bother fighting Calamitas in new games. I often find myself stopping before DoG because DoG is a long, boring battle with regular periods of boss invulnerability and very little ahead of it that I want to experience. Whatever comes beyond Calamitas must be TRYING HARDER than DoG, Yharon, and Calamitas, and it's got to suck. I can only imagine the frustration of trying to make the Calamity boss to end all Calamity bosses, but being gridlocked by escalating projectile spam, cheesy tricks to hinder the player, and by one-upsmanship with yourself.
Your Success is Based on Terraria
At the end of the day, I think maybe you've gotten turned around and you think Calamity is the most popular mod because of how amazing Calamity is, but in reality people play it because it adds more Terraria than most other mods. There's more weapons, more ores, and more bosses. Once you're done with the Moonlord, you go on a little longer. That's all people want, that's all people are looking for. New biomes would be nice if they were implemented properly and added to the progression of Terraria, but none of this stuff does that. Imagine if the Abyss were a post-moonlord biome that didn't need a super special suit to do anything in, and it was a place where you summoned a logical, well-built boss as part of the game's progression - and I don't mean yet another worm that drops equipment you may as well skip.
Maybe instead of focusing on trying to rebalance and reprogram Terraria, you should continue along the lines of adding more of the same Terraria experience to Terraria. Expand on the game as it is rather than trying to expand on Fabsol. Because as it is, your Fabsol expansions are making Terraria less convenient to play with friends, and you're making it harder for other mods to play along as well.
, he gets onto people all the time on his channel for messing with him and his work. Dokuro is likely going to be okay with his content being in a seperate mod, because you still need the mod to complete the experience in Calamity. The only reason it's being done is to allow the mod in its entirety (including his music, ffs) to be on TModLoader again, because contrary to whatever you may believe, music takes up a LOT of filespace. As a creator of songs, OGG files, which are the most compact files that can be used in a game, can still be huge when you factor in the size due to loops and due to the sheer amount of which are present in the mod.